Current Avatars:
blitzen - finish level 30 of blitzen mission
christmas tree - random when trying to get a cracker from the christmas tree
elf - click to use an elf costume (don't actually use it) when you have an elf pet
frozen - have a frozen food item in your inventory
ice - complete level 27 of blitzen
ice cold - buy an item from the frozen foods shop
ice fairy - use an ice fairy costume on your pet
ice fairy 2 - Play the Ice Fairy guess the item game
ice gnome - win over 50 battles against ice gnome
merry christmas - break a frozen advent wreath that someone has sent you
santa - win 340 times against the santa claws trading card
santa claws - do 140 santa claws quests
santa claws 2 - win against the santa claws trading card 230 times
seasonal - use a seasonal costume on your chibs
seasons greetings - random when donating an advent calendar item (it must be from the advent section of wishlists) to the pot of gold
snow fight - win 290 times against the snow fight trading card
snowman - complete 50 snowman quests
snowman twin - win over 200 battles against snowman twin
snowy - search for snow costume and click to view it
xmas 07 - feed a christmas candycane to your pet
dasher - attach a dasher to a snowy pet
tinsel - view a tinsel which is over 28 days old and is attached to a christmas pet

Retired Avatars:
advent
christmas
christmas eve
naughty